Java how to compare two generic type parameters

Question

The problem is to write generic method, with T constrained to extends Number. Only have a single field variable ArrayList. Have an add method that adds objects to the ArrayList, and then two methods, one that returns the largest in the ArrayList and the other the smallest. My problem is in the largest and smallest methods where I am comparing successive items in the list with each other searching for the largest and smallest, but they are of generic type T and I cannot use < and >, and compareTo() does not work either (I put both errors in the code provided). Can someone take a look and let me know where I am going wrong?

import java.util.ArrayList;

public class MyList<T extends Number> {
   private ArrayList<T> list;
   
   public MyList() {
      list = new ArrayList<T>();
   }
   
   public void add(T obj) {
      list.add(obj);
   }
   
   public T largest() {
      boolean onFirstObj = true;
      T largestVal;
      
      for (int i = 0; i < list.size(); i++) {
         if (onFirstObj) {
            largestVal = list.get(i);
            onFirstObj = false;
         }
         else {
            if (list.get(i) > largestVal) {
               largestVal = list.get(i);
            }
         }
      }
      
      return largestVal;
   }
   
   public T smallest() {
      boolean onFirstObj = true;
      T smallestVal;
      
      for (int i = 0; i < list.size(); i++) {
         if (onFirstObj) {
            smallestVal = list.get(i);
            onFirstObj = false;
         }
         else {
            if (list.get(i).compareTo(smallestVal) < 0) {
               smallestVal = list.get(i);
            }
         }
      }
      
      return smallestVal;
   }
}

Answer 1

The < and > operators can only be used with primitive types, and the Number class does not implement Comparable itself, so you can't use compareTo.

However if you add Comparable<T> as a type bound:

public class MyList<T extends Number & Comparable<T>> {
    ...
}

then you can use the compareTo method:

public T largest() {
    boolean onFirstObj = true;
    T largestVal = null;

    for (T t : list) {
        if (onFirstObj) {
            largestVal = t;
            onFirstObj = false;
        } else {
            if (t.compareTo(largestVal) > 0) {
                largestVal = t;
            }
        }
    }

    return largestVal;
}